The present invention relates to a transfer method for multi-color-forming materials and, in more detail, is concerned with a method of preparation of color proof used for the proofing operation in a photoengraving process.
In color printing, it is necessary to confirm beforehand the finish of printing for checking of color, tone, etc. obtained on finishing of printing at a step on the verge of plate-making, i.e. at the step of half-tone positive or half-tone negative prior to making the printing plate, or for checking of confirmation etc. by the orderer of the printing at the step of the proof sheet preparation. For this purpose, the color proof is prepared and the checking is made. With respect to this color proof, there have primarily been the surprint method, overlay method and transfer method hitherto depending on the forms thereof. In the surprint method, respective colored photosensitive liquors are coated in sequence onto a sheet of white support while repeating the printings from original plates of respective colors decomposed to form a multicolor image. Moreover, in the overlay method, printings are made from original plates of respective colors decomposed onto transparent supports coated with colored photosensitive liquors corresponding to respective colors to offer to the proofing by superposing them.
The invention relates to the transfer method described in Japanese Unexamined patent Publication No. Sho 61-286850, Sho 61-286858, etc. In this method, the exposure to light for imaging is given to a photosensitive sheet formed of a thermosensitive adhesive layer and a colored photosensitive layer in sequence onto a release surface provided on a support from which the thermosensitive adhesive layer is releasable and, after the development treatment, the image surface of the image-carrying sheet produced is superposed on a thermosensitive adhesive layer of an intermediate image-receiving sheet having only the thermosensitive adhesive layer on the release surface of the support from which the thermosensitive adhesive layer is releasable. By repeating then the concurrent heating and pressurizing, the images of image-carrying sheets are transferred in sequence onto the intermediate image-receiving sheet together with the adhesive layer to form a multicolor image. Further, this multicolor image on the intermediate image-receiving sheet is superposed on an arbitrary image receiver and, by heating and pressurizing concurrently, the multicolor image is formed on the arbitrary image receiver.
Concretely, yellow, magenta, cyan and black color images are formed, respectively, using four said photosensitive materials and these are transferred in sequence (regardless of the order) onto a same intermediate image-receiving sheet, thereby forming a multi-color image. Further, in order to bring this color proof close to the finish of printed matter, said intermediate image-receiving sheet with the multicolor image formed thereon is placed on an arbitrary image receiver, e.g. coated paper, art paper, newsprint paper or the like and, by heating and pressurizing concurrently, the multicolor image can be formed on the arbitrary image receiver. Moreover, if the image receiver is a material capable of being transferred with the thermosensitive adhesive layer and the colored photosensitive layer of the image-carrying sheets of the respective colors, it is also possible to transfer directly the image on the image-carrying sheet without using the intermediate image-receiving sheet.
When transferring the images in this way, however, as a factor affecting on the quality of finish, the adhesion between the image-carrying sheet and the image receiver when transferring the image can be mentioned. If the adhesion is poor, voids would be generated between the image layers of the respective colors resulting in the lowering in image density and the reduction in sharpness and saturation of the final finish due to those voids. Moreover, because the dot gain becomes a large amount, the color reproducibility of the transferred image is poor. As methods for improving the adhesion, there are a method to increase the thickness of thermosensitive adhesive layer and a method to modify the material in a way such as softening of the thermosensitive adhesive layer or the like. In the case of the former, however, if the final image receiver is paper, the image formed cannot reproduce the unevenness of paper to make the finish of printed matter different from original. Also, in the case of the latter, there arise such problems in operation as are apt to cause blocking and other problems.
As a method to improve the adhesion by the transfer method, there is a method of pressurizing under higher pressure or heating at high temperature. However, since said image-carrying sheet and said image-receiving sheet are all composed of soft materials, such problems that occurs are that both sheets which are superposed are warped when pressurizing under higher pressure or heating at high temperature by the use of a heat roller, that slipping-out of position is caused between said both sheets and that creases etc. generated on transporting through the heat roller may arise at the time of transfer treatment. Hence, a method is conceivable, wherein the image-carrying sheets are transferred in sequence onto the same intermediate image-receiving sheet under appropriate pressure and at appropriate temperature without causing the slipping-out of position to form the multicolor image and thereafter high pressure and high temperature are applied when transferring onto the final image receiver such as paper or the like. With this method, however, many voids are present between respective color layers since a plurality of colors are transferred onto the intermediate image-receiving sheet, it is hard for the air in voids to escape from the surface of intermediate image-receiving sheet since the support of the intermediate image-receiving sheet is mounted on the image receiver, the creases end up being generated on the image receiver on transporting through the heat roller since high pressure and high temperature are applied for the transfer, and so on. This cannot be said therefore to be a method satisfying the desired final finish.
As a result of diligent investigations on the transfer method to solve the problems aforementioned, the inventors have found a transfer method without said drawbacks leading to the completion of the invention.